Reflecting electric lamp



June 21, 1938. RElMAN ET AL 2,121,765

BEFLECTING ELECTRIC LAMP Filed Nov. 25, 1935 4 Sheets-Sheet l L ao BEFL ECT/NG kg/ZFACE WITH Po'lNT Sowecz AN /O0% v figFLzc-rlorv.

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POLAR 'I/v-rE/vs/TY Cum/E f or DIMENSIONAL FYLAMENT June 21, 193a.

Filed NOV. 25, 1935 (9 LAMI? fix/s c. K. REIMAN ET AL 2,121,765

REFLECTING ELECTRIC LAMP 4 Sheets-Sheet 2 N Inventor gawk 176M v by June 21, 1938 c:. K. REIMAN ET AL 2,121,765 REFLEQTiNG ELECTRIC LAMP Filed NOV. 25, 1935 4 Sheets-Sheet 3 M m 6 m I MM Z Q gag 2f D i A a/Wform g;

June 21, 1938. c. K. REIMAN ET AL. 2,121,765

Patented June 21, 1938 REFLECTING' ELECTRIC LAMP Clarence K. ltelman, Newton, and Joseph F. (look, In, West Roxbury, Mass, assignors to Birdseye Electric Company, a corporation of Delaware Application November 25, 1935, Serial No. 51,364

9 Claims. (01. 176-34) plane perpendicular to the main axis of the lamp,

of the area served by the lamp. For example,

with a single direct overhead lamp, designed to concentrate by means of reflecting surfaces the major part of the total light emitted from the filament in a circle, say 30 feet in diameter, it is' possible with the lamp herein disclosed to have the intensity of the illumination falling on the floor on every unit area within this 30 foot circle, substantially uniform. Not only is this resultnovel and an important improvement in the lighting art, when accomplished by means of the lamp only and without the use of any external reflecting device, but as far as we know it has never before been fully-accomplished even by the use of an external reflector.

In the past, uniform illumination on the working floor of a factory, for example, has been attempted by arranging the placement of the overhead external reflector lamps so that by overlapping the areas lighted by each lamp, an approximately uniform result was obtained. The desired intensity level is approximately that found directly under .one of the lamps, and the intensity along the floor-in any direction from a point directly under a single lamp gradually diminishes out to the edge .of the area lighted by the lamp in question. This effect may be shown graphically by means of a curve, the ordinates of which represent foot-candles, and the absclssae the distance in feet along the floor. The overlapping efiect of the other lamps bordering on this field is generally shown in the same manner, and then a final curve representing the sum of the ordinates (intensity from all sources) 7 is plotted for each of the floor points, and this may approximate a straight line. However, it never is a straight line and beyond this error of approximation this method is, upset by any pillar or column or part wall, or in fact any object that stands on the floor which will throw a shadow and eliminate the light from some one or more of the sources on a given area. Since the unit area to be lighted in a factory is frequently a bay, bounded on the corners by pillars,

and illuminated by a single lamp and reflector,

this consideration is of much importance.

In order to obtain'uniform illumination over the entire area illuminated by a single reflecting reflected, and this major portion of the total light so directed that it will fall on the floor near the outskirts of the lighted area. That is, it may be considered roughly that the. direct light will account for most of the illumination directly under the lamp, and since the area of the annular space between this center area and the periphery oi the lighted circle is greater than the center area, more light must be reflected into this annular space than falls directly into the center area. Thus a reflector to accomplish this result must completely ho'od the light source, that is, the filament must be well up inside the reflecting area, which is preferably a silver coating on the inside of the bulb itself, so that much more than half the total light will be gathered by the reflector and directed into the annular space. T

In designing a reflector to produce uniform illumination of the lighted area when this refleeting surface is to be part of the surface ]0f the bulb of the lamp, there are many considerations that do not enter into the design of external reflectors. Thus the reflector must obviously be of limited size, and the shape must be such that it is physically possible to incorthan is required for the larger and much more distant external reflector, the final resulting refiector is of diflerent shape from an external reflector designed to attempt the same effect.

We have discovered that it is possible 8.0-

which when made contiguous with part of the surface of the bulb enclosing the filament, will produce uniform illumination over the illumicurately to design a series of reflecting surfaces upon the distance from the plane illuminated, 50

and (0) upon the angle of cut-oifthat is, the angle between the vertical line from the center of the lamp to the plane and the outside of the cone of light shining from the lamp to the plane. However, the'distribution will, within reasonable limits, be substantiially uniform regardless of the magnitude of these factors. This relation holds true except when a wide angle lamp is moved very close to the plane-at which time a noticeable excess of light falls on the area directly under the lamp, and at great distances non-uniformity is also apparent. For working distances of from 3 to 20 feet, a substantially uniform illumination may be derived from a lamp constructed in i 'Jrdance with our invention.

Other features and advantages of the invention will be understood and appreciated from the following description of a preferred embodiment thereof. selected for purposes of illustration and shown with explanatory diagrams in the accompanying drawings, in which,-

Fig. 1 is a diagram showing the shape of reflecting surfaces which, under different indicated conditions, produce uniform light distribution and includes a polar intensity diagram.

Fig. 2 is a polar intensity diagram of light emanating from a compactly arranged filament without any reflector.

Figs. 3, 4 and 5 are diagrams showing the curve of the reflecting surfaces and polar intensity diagrams of lamps having respectively 60, 65 and 30 of cut-oil.

Fig. 6 is a view in elevation, partly in section,

of a lamp embodying our invention, the neck portion being broken away, and

Fig. '7 is a, diagram showing the curve of the reflecting surfaces and polar intensity diagram of a lamp having a 45 angle of cut-off.

The diagrams above referred to show traces in the plane of the paper of one half of the reflector,

producing uniform illumination, when a theoretical light source is located at its center at the point 0, and when the light source is a commercial filament having an emission intensity curve approximately that shown in Fig. 2.

In Fig. 1 we have shown the traces of two similar reflectors, the two curves overlapping throughout much of their extent, and both being designed to produce uniform illumination when the light source is centered at the point 0. One curve represents the theoretical case where the light source is considered to be a point with spherical emission intensity and with a reflection factor of 100%. This curve is shown in dot and dash lines and is marked Reflecting surface with point source and 100% reflection. The other curve is the practical case where a dimensional filament having a non-uniform emission intensity is employed with a reflecting surface having a reflection factor of 70%. This curve is shown in Fig. l in full lines and is marked Reflecting surface with dimensional source and 70% reflection. It will be understood that the only important difference between these curves occurs near the apex or closed part and, as will be seen hereinafter, this part of the reflector is less important and less critical in the requirements of its design than the side walls of the reflector which actually hood the light source.

In Fig. 1 and in the other diagrams of the drawings there is shown a cross-hatched area which represents the luminous intensity on polar coordinates necessary'in order to satisfy the requirement of uniform illumination for the cutofi angle shown, that is 30 in Fig. 1. Since the bulb and reflector are symmetrical about the main axis, this diagram is the same in any plane passing through the axis. The length of any radius from the point 0 to the edge of the cross hatched area. is proportional to the intensity of the light sent in the direction of that radius from the combined sources of the direct and reflected light.

The several reflectors indicated by their half traces in the various figures of the drawings are designed for uniformly lighting a given area and vary only in their cut-off angles which determine the size of the circle to be illuminated. We have found that efficient lamps may be constructed having a cut-off angle of any magnitude between approximately 30 and 67. For smaller angles the length of the reflector, and therefore the bulb, becomes impractical for commercial construction, and for larger angles the light reflected from one side of the reflector strikes the lower edge of the opposite side and interferes with the desired results.

In Fig. '7 is depicted the data of a reflecting lamp having a cut-off angle of 45 and in this figure is indicated a group of light rays starting from the source 0, striking the surface of the reflector, and then passing down to the plane being illuminated. It will be noted that the light rays striking the lowest part or skirt of the reflectornearest the transmitting part of the bulb-are thrown farthest away from the center of the circle to be illuminated, while rays reflected by parts of the reflector nearest its apex are reflected in a direction bringing them closer and closer to the center of the light circle. Rays from the apex, in the case of a point source of light, would of course be reflected downwardly and perpendicularly, although the filament and the mount somewhat interfere with the efficiency of the reflection in this area since most of the rays covered by the hooding reflector must be directed toward the more distant part of the circle to be illuminated. In orderto produce uniform illumination, it is apparent that the skirts of the reflector must be designed with especial care. From the same reasoning it is apparent that, conversely, the curve of the reflecting surface nearer its apex is of less importance.

Referring again to the fact, shown in Fig. '7, that the reflected rays cross each other, those from the base of the skirt of the reflector being directed to the circumference of the circle, we

wish to stress the point that this has been the case in every reflector answering all the requirements of this problem of uniform illumination that we have designed. It is so in each of the specific cases shown in the drawings.

We have examined the shape of the traces of curves designed as reflectors to satisfy the requirements of this problem of uniform illumination, and in every case we have found the major part of this trace, and always including the skirts of the reflector and continuously to a point near the apex of the reflector, to be half of a true catenary, having a mathematical formula or equationwhere X and Y are the rectangular coordinates, (e) the base of the Naperian logarithm, and (a) the distance along the axis Y from the X-axis to the point where the curve cuts the Y axis. The part of the curve which is a true catenary ends, in each case, at some point marked I 0 on the several figure-sof these traces shown in the drawings, and does not extend to the apex of the reflector itself. The rest of the curve from this point III to the apex of the reflector is some other not readily determined curve, and although we have attempted unsuccessfully to determine its mathematical equation, we have not been greatly interested in this part of the curve, since its usefulness and accuracy is of much less importance to a proper solution of the problem of uniform illumination than the skirts of the curve, and for the reasons indicated above. Thus the two half catenaries, when in proper position with respect to the trace of the complete reflector, appear to be connected by a smooth curve which is of course perpendicular to the axis of reflection where it crosses it. 7

It is necessary to point out that the entire curve of the reflector is not a catenary, nor is the entire curve with the exception of the part near the apex. Nor is the axis of the reflector the axis of the catenary. The axis of the catenary forming most of one, side of any of the reflectors shown in. the drawings is in all cases the line NN, perpendicular to the trace of the reflector at the point where the catenary curve ends. The axis N--N forms an angle with the axis of the reflector marked A on the drawings. The catenary portion of the traceforming the other half of the reflector has a similar axis, and if these two half curves are moved in relation to each other, until these two axes coincide and so that the points where the two half catenaries cut their respective axis also coincide, there will be formed a full catenary. Thus the reflecting surface of our invention may be described as a surface of revolution generated by revolving about the axis of the reflector, a curve,'the major portion of which is a catenary, the reflector however, not being a catenary of revolution, about its own axis, over any part of its surface.

In Fig. 6 we have illustrated a reflecting electric lamp designed in accordance with the principles above described and havinga reflecting surface for producing uniform illumination and a cut-off angle of 65. The main body of the bulb 20 is a surface of revolution formed by revolving a half catenary curve about a vertical axis in the manner above described. The bulb is provided with an internal silver reflecting coating extending from a line in the neck 22 and substantially to the ,line of maximum bulb diameter. The curved end of the bulb 2| may be of clear glass or it may be frosted. The filament mount 23 is of the usual construction and supports a filament 25 by means of lead-in and supporting wires 24 in such relation to the reflecting surface as to determine the cut-offangle of the lamp at 65. The filament 25 is shown as'being of the coiled type and is compactly disposed in a V-shaped design so that it serves as a concentrated light source. A circular dished reflector 26 is located in theneck of the bulb in position to act as the apex of the catenary curve, and its reflecting curvature is substantially continuous with that of the bulb itself. The design and location of the reflecting disk 26 is not critical because as already pointed out it occupies a less critical part of the reflecting area.

In order to avoid reflected images and shadows of the filament on the illuminated area, we prefer to design and construct these-bulbs so that the reflecting surfaces are also diffusing surfaces, for example by first frosting the surface of the catenary area and then silvering over this frosted area as described in application. Serial No. 47,581 of Birdseye and Deren. A diffused light may also be obtained by frosting the clear non-reflecting part of the bulb, although this does introduce a measurable loss in efficiency.

A catenary reflecting bulb for indirect lighting may be designed in more than one way. For example, the same type of bulb used for overhead direct lighting may be used in the reverse posi- 1. An electric lamp comprising a bulb and a symmetrical reflecting surface associated therewith of such shape that any plane passed through the axis of the reflecting surface will be cut by it in a curve symmetrical with respect to the axis, the major portion of said curve being a catenary located with its axis at an acute angle to the axis of the reflecting surface.

2. An electric lamp comprising a light source and a bulb havinga reflecting surface associated therewith of such shape that any plane passed through the axis of the reflecting surface will present the trace of half an inwardly concave catenary on each side of the axis described about an axis inclined to the axis of the bulb.

3. An electric lamp comprising a light source and a bulb having associated therewith a reflecting surface which is the locus generated by half of a catenary which is inwardly concave revolved about an axis which intersects the axis of the catenary.

4. A reflecting electric lamp comprising a bulb having a portion formed as a surface of revolution by a catenary curve which is inwardly concave about the axis of the reflecting surface, said axis intersecting the axis of the catenary curve,

a reflecting medium applied to said surface of revolution, and a concentrated light source.

' 5. A reflecting electric lamp comprising a bulb having reflecting side walls formed as a surface of revolution by a catenary curve revolved about the axis of a lamp and being concave with respect to said axis, and a light source hooded by said surface, the axis of the catenary intersecting the axis of the lamp at a point in advance of the light source.

6. In an electric lamp, a bulb having a reflecting surface of revolution whose generatrix is one branch of an inwardly concave catenary curve, and whose axis intersects the axis of said catenary curve at a point within said surface of revolution.

'7. In an electric lamp, a bulb having a reflecting surface of revolution whose generatrix is one branch of a catenary. curve, and of which the axis intersects the axis of said catenary curve at a point not less than nor more than of the distance from the vertex of said surface to the open end thereof, said angle of intersection branch of a catenary curve whose axis intersectsment light source and a bulb having a reflecting surface upon a portion of its walls which is formed as a surface of revolution about the axis of the bulb by revolving as the generatrix a compound curve of such shape that the filament is completely hooded by the reflecting surface gen- .erated, said curve in all portions being concave toward the bulb axis, and the lower and the outer and major portion being a half catenary disposed with its axis intersecting the bulb axis at an acute angle.

CLARENCE K. REIMAN. JOSEPH F. COOK, JR. 

